Datasheet

LTC1772B
8
1772bfa
APPLICATIONS INFORMATION
The basic LTC1772B application circuit is shown in Figure 1.
External component selection is driven by the load require-
ment and begins with the selection of L1 and R
SENSE
(= R1).
Next, the power MOSFET, M1 and the output diode D1 is
selected followed by C
IN
(= C1)and C
OUT
(= C2).
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
With the current comparator monitoring the voltage de-
veloped across R
SENSE
, the threshold of the comparator
determines the inductors peak current. The output current
the LTC1772B can provide is given by:
I
R
I
OUT
SENSE
RIPPLE
=
0 105
2
.
where I
RIPPLE
is the inductor peak-to-peak ripple current
(see Inductor Value Calculation section).
A reasonable starting point for setting ripple current is
I
RIPPLE
= (0.4)(I
OUT
). Rearranging the above equation, it
becomes:
R
I
SENSE
OUT
=
0 0875.
for Duty Cycle < 40%
However, for operation that is above 40% duty cycle, slope
compensation effect has to be taken into consideration to
select the appropriate value to provide the required amount
of current. Using Figure 3, the value of R
SENSE
is:
R
SF
I
SENSE
OUT
=
()
(. )0 0875
100
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies permit the use
of a smaller inductor for the same amount of inductor ripple
current. However, this is at the expense of effi ciency due
to an increase in MOSFET gate charge losses.
The inductance value also has a direct effect on ripple
current. The ripple current, I
RIPPLE
, decreases with higher
inductance or frequency and increases with higher V
IN
or
V
OUT
. The inductors peak-to-peak ripple current is given
by:
I
RIPPLE
=
V
IN
V
OUT
fL
( )
V
OUT
+ V
D
V
IN
+ V
D
where f is the operating frequency. Accepting larger values
of I
RIPPLE
allows the use of low inductances, but results
in higher output voltage ripple and greater core losses.
A reasonable starting point for setting ripple current is
I
RIPPLE
= 0.4(I
OUT(MAX)
). Remember, the maximum I
RIPPLE
occurs at the maximum input voltage.
The ripple current is normally set such that the induc-
tor current is continuous down to approximately 1/4 of
maximum load current. This results in:
I
R
RIPPLE
SENSE
003.
This implies a minimum inductance of:
L
MIN
=
V
IN
V
OUT
f
0.03
R
SENSE
V
OUT
+ V
D
V
IN
+ V
D
(Use V
IN(MAX)
= V
IN
)
A smaller value than L
MIN
could be used in the circuit;
however, the inductor current transitioning from continuous
to discontinuous will occur at a higher load current.
Power MOSFET Selection
An external P-channel power MOSFET must be selected
for use with the LTC1772B. The main selection criteria for
the power MOSFET are the threshold voltage V
GS(TH)
and
the “on” resistance R
DS(ON)
, reverse transfer capacitance
C
RSS
and total gate charge.